ENZYMEare available. Generally the active site cavity is
defined by comparing the superposition of active to
that of inactive molecules.
receptor-mediated endocytosis Cells use receptor-
mediated endocytosis for ingestion of nutrients, hor-
mones, and growth factors, a method where specific
molecules are ingested into the cell; the specificity
results from a RECEPTOR-LIGAND (a molecule or ion
that can bind another molecule) interaction. Other lig-
ands that can be ingested include toxins and lectins,
viruses, and serum transport proteins and antibodies.
Receptors, a specific binding protein such as clathrin
on the plasma membrane of the target tissue, will
specifically bind to ligands on the outside of the cell.
An endocytotic process results: the cell folds inward
with a portion of the plasma membrane, and the result-
ing clathrin-coated pit is pinched off, forming a mem-
brane-enclosed bubble or vesicle, called an endosome.
After entering the cytoplasm, the endocytotic vesicle
loses its clathrin coat and the ligand (multiple ligands
can enter the cell in the same coated pit) is ingested.
The receptor can be recycled to the surface by vesicles
that bud from the endosome targeting the plasma mem-
brane. After these recycling vesicles fuse with the
plasma membrane, the receptor is returned to the cell
surface for binding and activity once more.
receptor potential A change in a neuron’s mem-
brane potential (a change in voltage across the recep-
tor membrane) caused by redistribution of ions
responding to the strength of the stimulus. If the
potential is high enough, an action potential will be
fired in an afferent neuron. The more action poten-
tials fired, the more neurotransmitters released, and
the stronger the signals reaching the brain. Also called
an end-plate potential.
redox potential Any OXIDATION-REDUCTION(redox)
REACTIONcan be divided into two half reactions: one
in which a chemical species undergoes oxidation and
one in which another chemical species undergoes
reduction. If a half-reaction is written as a reduction,
the driving force is the reduction potential. If the half-
reaction is written as oxidation, the driving force is the
oxidation potential related to the reduction potential
by a sign change. So the redox potential is the reduc-
tion/oxidation potential of a compound measured
under standard conditions against a standard reference
HALF-CELL. In biological systems, the standard redox
potential is defined at pH = 7.0 versus the hydrogen
electrode and partial pressure of hydrogen = 1 bar.
See alsoELECTRODE POTENTIAL.reducing agent The reactant that donates its elec-
trons and in turn becomes oxidized when another sub-
stance is reduced.
See alsoOXIDATION-REDUCTION REACTION.reductase SeeOXIDOREDUCTASE.reduction The part of a redox reaction where the
reactant has a net gain of electrons and in which a dif-
ferent reactant must oxidize (lose electrons).
See alsoOXIDATION.reductive elimination The reverse of OXIDATIVE
ADDITION.refractory material/metal Any material that with-
stands high heat or a metal with a very high melting
point, e.g., clay, tungsten, molybdenum, tantalum, nio-
bium, chromium, vanadium, and rhenium.regioselectivity, regioselective A regioselective reac-
tion is one in which one direction of bond making or
breaking occurs preferentially over all other possible
directions. Reactions are termed completely (100 per-
cent) regioselective if the discrimination is complete, or
partially (x percent) if the product of reaction at one
site predominates over the product of reaction at other
sites. The discrimination may also semiquantitatively
be referred to as high or low regioselectivity. (Origi-
nally the term was restricted to ADDITION REACTIONs
of unsymmetrical reagents to unsymmetrical alkenes.)
In the past, the term regiospecificitywas proposed
for 100 percent regioselectivity. This terminology is notregioselectivity, regioselective 235